1. Field of the Invention
The present invention relates to a cover plate for a display device using a liquid crystal display (LCD) or the like and intended for use as a display unit for various electronic devices, such as portable telephones, a method of making such a cover plate, a display device employing such a cover plate, and a low-reflection structure.
2. Description of the Related Art
Various current electronic devices are provided with a display unit including a liquid crystal display (LCD) as a display panel, and the display panel is not exposed to protect the same from water, dust, external forces and the like. A window paned with a cover plate, such as a transparent plastic plate, is disposed on the outer side of the display panel with a space between the cover plate and the display panel (JP-A No. 66859/1995). For example, a portable telephone has a case formed by molding a resin or the like and provided with a window corresponding to a display unit and paned with a transparent cover plate, and a liquid crystal display panel mounted on a base plate is disposed on the inner side of the window to protect the liquid crystal display panel.
The additional, protective cover plate lying between the display panel and the user reflects external light to make it difficult for the user to see an image displayed on the display panel, and reflects image light emitted by the display panel to reduce light utilization efficiency. Thus, while the cover plate achieves protective purposes, the same deteriorates visibility.
To solve such a problem, a method proposed in JP-A No. 127852/2001 forms a single-layer antireflection film or a multilayer antireflection film consisting of a layer having a high refractive index and a layer having a low refractive index by a vapor deposition process, a sputtering process or a coating process on a cover plate, and a method proposed in JP-A No. 325202/1995 applies an antireflection film to a cover plate. The antireflection films formed by a vapor deposition process, a sputtering process or such have problems in stability and the ratio of good antireflection films, and cannot be manufactured at a high productivity because thin films having a controlled refractive index must be formed by one or a plurality of cycles of a batch process. The antireflection films formed by a coating process must be formed individually on cover plates and hence cannot be formed at a high productivity.
It is an object of the present invention to provide a cover plate to be disposed in front of a display panel, capable of reducing reflection of light, of maintaining its function and of improving visibility, a method capable of stably making the cover plate at a high productivity at a low fraction defective, a display device provided with the cover plate, and a low-reflection structure.
According to the present invention, a cover plate for a display device provided with a display panel, disposed so as to cover the display panel has a transparent base having a surface facing the display panel and provided with reflection-reducing minute irregularities; wherein the reflection-reducing minute irregularities of the transparent base consist of numerous projections and numerous valleys lying between the projections, and meet a condition expressed by:
PMAXxe2x89xa6xcexMIN
where PMAX is the maximum distance between the tips of the adjacent projections, and xcexMIN is the minimum wavelength in a visible region of visible light in a vacuum, and the sectional area ratio of the total area of sections of the projections of the transparent base in a plane parallel to the transparent base in the plane increases continuously and gradually in a direction from the tips of the projections toward the bottoms of the valleys.
In the cover plate according to the present invention, the transparent base meets a condition expressed by:
PMAXxe2x89xa6xcexMIN/nb
where nb is a refractive index of the transparent base.
In the cover plate according to the present invention, the minimum wavelength xcexMIN in the visible region in a vacuum is 380 nm and hence PMAXxe2x89xa6380/nb.
In the cover plate according to the present invention, the projections and the valleys of the reflection-reducing minute irregularities have substantially the same shapes, respectively.
In the cover plate according to the present invention, the projections and the valleys of the reflection-reducing minute irregularities are arranged at the same pitches.
In the cover plate according to the present invention, the projections of the reflection-reducing minute irregularities meet a condition expressed by:
HMINxe2x89xa70.2xcexMAX
where HMIN is the minimum height of the projections.
In the cover plate according to the present invention, the projections of the reflection-reducing minute irregularities meet a condition expressed by:
HMINxe2x89xa70.4xcexMAX
In the cover plate according to the present invention, the projections of the reflection-reducing minute irregularities meet a condition expressed by:
HMIN/PMAX=1/2 to 4/1
The refractive index between the back surface of the cover plate and air can be made to change continuously and gradually instead of discontinuously and sharply. Since the reflection of light at the interface between substances is caused by the sudden change in refractive index, the reflection of light by the back surface of the cover plate can be reduced by making the refractive index at the back surface of the cover plate change continuously and gradually. Consequently, light rays reflected by the back surface of the cover plate among light rays reflected toward the user by the front and the back surface of the cover plate among external light rays that affect the visibility of a displayed image, more strictly, visible light fallen on the cover plate, can be eliminated and hence the total amount of the light rays reflected by the cover plate can be reduced accordingly. Thus, the deterioration of visibility by the reflected external light can be reduced. Since the specific minute irregularities are formed only on the back surface of the cover plate and is not formed on the front surface of the same, the reduction of the reflection-reducing effect of the reflection-reducing minute irregularities due to the abrasion of the reflection-reducing minute irregularities and soiling of the reflection-reducing minute irregularities with oil, i.e., filling of the valleys with oil, does not occur.
In a display device provided with this cover plate, reflection of display light (image light) emitted by a display panel disposed behind the cover plate by the back surface of the cover plate can be reduced and the cover plate has a high transmittance, whereby the reduction of contrast in the displayed image and the resultant whitening due to multiple reflection of the display light between the front and the back surface of the cover plate can be prevented and the visibility of the displayed image can be improved. Since visibility is improved, the power demand of the back light or front light of the liquid crystal panel or the power demand of the display panel, such as an EL panel can be reduced, which contributes to the reduction of the power demand and power consumption of the related electronic devices.
The reflection-reducing effect of the specific minute irregularities can be enhanced by determining the relation between PMAX and xcexMIN by using the refractive index nb of the transparent base, taking into consideration the wavelength of light which becomes shorter when the light enters the transparent base from a vacuum.
According to the present invention, a cover plate making method of making a cover plate for a display device, having a transparent base having one surface provided with reflection-reducing minute irregularities includes the steps of: forming a resist film on a glass plate, forming a resist pattern by subjecting the resist film to exposure and developing processes for patterning; forming a mold provided with mold minute irregularities by etching the glass plate using the resist pattern as a mask; and forming the transparent base provided with the reflection-reducing minute irregularities by using the mold provided with the mold reflection-reducing minute irregularities.
In the cover plate making method according to the present invention, the resist pattern is formed by exposing the resist film by electron beam lithography, and developing the exposed resist film for patterning.
In the cover plate making method according to the present invention, the resist pattern is formed by exposing the resist film to light by laser beam lithography, and developing the exposed resist film for patterning.
In the cover plate making method according to the present invention, the transparent base provided with the reflection-reducing minute irregularities is molded by an injection molding process using the mold provided with the mold minute irregularities.
The cover plate making method using the mold is able to mass-produce easily cover plates provided with the specific minute irregularities.
The cover plate making method is capable of simultaneously forming the transparent base and the reflection-reducing minute irregularities of the same material, and of mass-producing the cover plates at a low manufacturing cost.
According to the present invention, a display device includes: a display panel; and a transparent base covering the display panel and having a surface facing the display panel and provided with reflection-reducing minute irregularities; wherein the reflection-reducing minute irregularities of the transparent base consist of numerous projections and numerous valleys lying between the projections, and meet a condition expressed by:
PMAXxe2x89xa6xcexMIN
where PMAX is the maximum distance between the tips of the adjacent projections, and xcexMIN is the minimum wavelength in the visible region in a vacuum, and the sectional area ratio of the total area of sections of the projections of the transparent base in a plane parallel to the transparent base in the plane increases continuously and gradually in a direction from the tips of the projections toward bottoms of the valleys.
The display device according to the present invention is able to enjoy the foregoing effect of the cover plate. Consequently, the visibility of displayed images is improved, and the energy consumption and power demand of the display device can be reduced.
According to the present invention, a low-reflection structure includes a transparent base having one surface provided with reflection-reducing minute irregularities, and an antireflection film formed on the other surface of the transparent base; wherein the reflection-reducing minute irregularities of the transparent base consist of numerous projections and numerous valleys lying between the projections, and meet a condition expressed by:
PMAXxe2x89xa6xcexMIN
where PMAX is the maximum distance between the tips of the adjacent projections, and xcexMIN is the minimum wavelength in the visible region in a vacuum, and the sectional area ratio of the total area of the sections of the projections of the transparent base in a plane parallel to the transparent base in the plane increases continuously and gradually in a direction from the tips of the projections toward bottoms of the valleys.
In the low-reflection structure according to the present invention, the reflection-reducing minute irregularities reduce reflection of light by the surface provided with the reflection-reducing minute irregularities, and the antireflection film formed on the other surface reduces reflection of light by the surface covered with the antireflection film. Thus the reflection-reducing ability of this low-reflection structure is higher than that of a structure having only one surface provided with the reflection-reducing minute irregularities. Reflection of light, which is a phenomenon that occurs when refractive index changes suddenly and discontinuously, can be reduced by the reflection-reducing minute irregularities because the refractive index between a surface of a medium and air can be made to change continuously and gradually by forming minute irregularities of sizes smaller than the wavelengths of light, which must be prevented from reflecting, in the surface of the medium.
Since the irregularities of the present invention reduce the reflection of light by the surface of the medium by making the refractive index change gradually at the interface between the surface of the medium and air instead of preventing specular reflection of light by a satin-finished surface for glare prevention, transmittance can be improved according to the reduction of reflectance. The antireflection film formed on the surface other than the surface provided with the reflection-reducing minute irregularities is not an antiglare film that reduces specular reflection and hence the antireflection film improves transmittance according to the reduction of reflectance. Accordingly, when the low-reflection structure is used as a cover plate for covering the display unit of a display or the like, the low-reflection structure improves the visibility of displayed images and improves display light utilization efficiency.
According to the present invention, a reflection-reducing structure includes a transparent base having a first surface provided with reflection-reducing minute irregularities, and a second surface provided with antiglare irregularities of sizes greater than the wavelengths in the visible region; wherein the reflection-reducing minute irregularities of the transparent base consist of numerous projections and numerous valleys lying between the projections, and meet a condition expressed by:
PMAXxe2x89xa6xcexMIN
where PMAX is the maximum distance between the tips of the adjacent projections, and xcexMIN is the minimum wavelength in the visible region in a vacuum, and the sectional area ratio of the projections of the transparent base in a plane parallel to the transparent base increases continuously and gradually in a direction from the tips of the projections toward the bottoms of the valleys.
In the low-reflection structure according to the present invention, the reflection-reducing minute irregularities reduce reflection of light by the first surface, and the antiglare irregularities reduce reflection of light by the second surface. Thus, the reflection-reducing ability of this low-reflection structure is higher than that of a low-reflection structure provided with minute irregularities only in one of its surfaces.
The low-reflection structure may be used for the same reflection-reducing effect with either the first surface provided with the reflection-reducing minute irregularities or the second surface provided with the antiglare irregularities facing outside. Thus, the user may optionally decide which of the first and the second surface to use as an outer surface. However, in view of the durability of reflection-reducing effect and the effective utilization of the antiglare ability, the reflection-reducing structure is used with the first surface provided with the reflection-reducing minute irregularities facing inside and the second surface provided with the antiglare irregularities facing outside; that is, the low-reflection structure is used such that the first surface provided with the reflection-reducing minute irregularities serves as an inner surface, i.e., a back surface which is not exposed outside and less possible to be damaged than a front surface, and the second surface provided with the antiglare irregularities serving as an outer surface, i.e., a surface exposed outside and possible to be damaged. When the low-reflection structure is used in such a manner, the reflection-reducing effect of both the irregularities is hardly reduced by damaging, and the reflection-reducing effect is satisfactorily durable, and the front surface, i.e., the outer surface, has an antiglare ability.
Reflection of light, which is a phenomenon that occurs when refractive index changes suddenly and discontinuously, can be reduced by the reflection-reducing minute irregularities because the refractive index between a surface of a medium and air can be made to change continuously and gradually by forming minute irregularities of sizes smaller than the wavelengths of light rays which must be prevented from reflecting in the surface of the medium.
Since the reflection-reducing minute irregularities reduce the reflection of light on the surface of the low-reflection structure by making the refractive index change gradually at the interface between the surface of the low-reflection structure and air instead of reducing reflection of light by specular reflection of light by the antiglare irregularities, transmittance can be improved according to the reduction of reflectance. Therefore, although the low-reflection structure is provided with the antiglare irregularities, when the reflection-reducing structure is used as a cover plate for covering the display unit of a display or the like, the reflection-reducing structure improves the visibility of displayed images and improves display light utilization efficiency.